1. Field of the Invention
The present invention relates to mounting systems for semiconductor devices as used in electronic equipment, and more particularly to mounting devices for high-voltage devices.
2. Description of Related Art
Certain semiconductor devices are designed to handle relatively high voltages in a compact space. For example, semiconductor devices that are exposed to RMS voltages greater than 100 VAC, such as 265 VAC or 415 VAC, are often mounted in electronic power supplies and the like. These devices may dissipate relatively large amounts of power, and are accordingly often mounted to heat sinks or like devices as well as being electrically connected to electronic equipment of various types.
Many such semiconductor devices for power applications are commonly available in the JEDEC standard TO-220 package (www.jedec.org). An exemplary TO-220 package 110 is shown in FIG. 2 in conjunction with a mounting system according to the invention. The TO-220 package has a body 112 with leads 114 exiting the package 110 on one side, and a mounting flange 116 protruding from the other side of body 112. Internal to package 110, a semiconductor die (not shown) is attached to a heat spreader that is integral with the mounting flange 116. The mounting flange 116 has a hole 118 for mounting the package 110. High-voltage semiconductor devices may also be available in various other packages similar to the TO-220 package.
Some prior art methods of mounting TO-220 packages and like devices involve attaching the package to a heat sink or other substrate using a threaded fastener (e.g., a screw) or rivet that passes through the mounting hole 118. While screw mounting may be convenient for low-volume production, the use of screws often involves assembling multiple other components besides the screws and the component to be mounted, such as threaded nuts or plates and washers of various types. The assembly of multiple components may substantially increase the cost of mounting the device. In addition, many high-voltage applications require electrical isolation between the device and its mounting substrate, which may further increase mounting costs because of the addition of insulating washers and bushings. Screws are also subject to loosening due to vibration, and assembly of screw-mounted components can be difficult to automate. Tool clearance, which uses valuable space, should also be provided in equipment using threaded fasteners. Some of these disadvantages may be avoided by using rivets instead of screws, but rivets may involve higher component costs and can make it difficult to rework or replace the mounted device.
In general, whether screws, rivets, or other fasteners are used, the system of mounting the package to a substrate using a mounting hole through a protruding flange suffers from other disadvantages. The single mounting point may not prevent a semiconductor device from rotating around its mounting hole, making it more difficult to align and attach an assembled device to an electronic component such as a printed circuit board. The clamping force provided by the fastener is not centered over the body of the package, where it would be most effective for transferring heat from the body. Instead, the clamping force is centered on the flange which increases the path length and thermal resistance between the heat source (the semiconductor die) and the heat sink. Electrical insulating materials used as washers or bushings with the fastener may also increase thermal resistance. Increased thermal resistance, in turn, may cause undesirably high operating temperatures for the semiconductor die.
Other prior art mounting systems provide a heat sink configured as a spring clip that may be clipped directly to the package body. These systems also suffer from disadvantages. Heat sink materials should be highly thermally conductive, such as are aluminum and copper, but most thermally conductive metals are relatively soft and make poor spring materials. Accordingly, the clamping force provided by a spring clip made with thermally conductive metals is too low for optimum heat transfer. More exotic materials may be used for spring clip-type heat sinks, such as beryllium copper alloys, but these materials are relatively expensive, and are less thermally conductive than cheaper materials such as aluminum or copper. Spring clips, which typically are configured in arcuate shapes, are generally not capable of achieving good thermal contact over the entire flat side of a semiconductor body. Spring clip-type heat sinks suffer from the further disadvantage of requiring the package leads to support the package and the entire heat sink, which can contribute to failure of the semiconductor device in vibration environments.
Still other prior art systems use a clip to compress the body of a semiconductor package against a flat surface of a heat sink. This avoids the disadvantages of using an extending flange as the mounting surface, while enabling the spring clip to be made of a more suitable spring material. The spring clip in these designs need not be a good thermal conductor because it no longer functions as the primary conductive thermal path. A drawback of prior art systems of this type is that they have been relatively bulky. One prior art system requires a screw for attaching the spring clamp to the heat sink. The use of a screw attachment brings associated disadvantages similar to those mentioned above. Another prior art system avoids the use of a screw by wedging one end of a spring clip into a special slot of the heat sink while using the other end of the spring clip to compress a semiconductor device against the heat sink. This configuration is also relatively bulky, and requires a specially shaped heat sink and/or spring clip for each different application.
None of the aforementioned prior-art systems provide for precise alignment of the semiconductor device relative to the heat sink, which may make it difficult to automate assembly of the system to printed circuit boards or other components. Furthermore, each of the aforementioned systems requires an undesirable trade-off between high-voltage safety and operating temperature. High-voltage safety may be improved by enclosing the semiconductor package in an insulating material. Prior-art systems commonly use a silicone rubber tubing as the insulating material, but this material suffers from the disadvantage of being a poor thermal conductor, which contributes to higher operating temperatures. Silicone rubber is also soft and subject to cut-through at the pressure line of a spring clip, which can lead to electrical shorting between the spring clip and the body of the semiconductor package.
It is desired, therefore, to provide a mounting system for a semiconductor device that overcomes the limitations of prior-art mounting systems. The mounting system should be inexpensive, compact, easy to assemble using automated processes, readily disassembled for rework or repair, versatile (i.e., a standardized system that may be readily used with various different configurations of heat sinks), rugged, reliable, and safe for use with high-voltage devices, while providing superior heat transfer from the device to the heat sink.
The present invention provides a mounting system for a semiconductor device that overcomes the limitations of prior-art mounting systems, and is particularly suitable for mounting high-voltage semiconductor devices to heat sinks in a compact fashion.
The mounting system is for mounting a semiconductor package such as a JEDEC TO-220 to a heat sink. The system may be applied to any semiconductor package having a semiconductor die, a heat spreader, a body of non-conductive material around the semiconductor die, and a plurality of leads extending from the body. The package may optionally include a mounting flange extending from the body. The mounting flange may have a through hole. The heat sink may be of any suitable configuration that includes a sheet of thermally conductive material in at least a portion of the heat sink to serve as a mounting surface.
The mounting system comprises two primary elements: a rigid retainer and a pressure clamp. The mounting system optionally includes a thermally conductive, electrically insulating spacer. The rigid retainer comprises a block of non-conductive material having a recess open to at least two adjacent faces of the block of non-conductive material and configured to expose at least a portion of the heat spreader when the semiconductor package is disposed in the recess with the plurality of leads extending from the retainer. The pressure clamp is configured for disposing against an exterior of the retainer opposite to the body and attaching to the heat sink, for compressing the package against the heat sink.
In an embodiment of the invention, the pressure clamp is a spring clip configured for disposing partly around the retainer, the semiconductor package, and the heat sink, such as a U-shaped clip. The spring clip has a first clamping arm for disposing against the exterior of the retainer resiliently connected to a second clamping arm for disposing against the heat sink. The spring clip may be made of any suitable spring material, such as spring steel. The spring clip may be provided with a protrusion, such as a stamped barb-like projection, in the second clamping arm for disposing in a recess of the heat sink, for locking the clip in a desired position relative to the heat sink. In the alternative, or in addition, the spring clip may have a protrusion or protruding portion in the first clamping arm for disposing in a recess or recessed portion of the retainer, for locking the clip in a desired position relative to the retainer.
In an alternative embodiment, the pressure clamp is a rigid bar configured for disposing against the exterior of the retainer and attaching to the heat sink with at least ail two fasteners, such as threaded fasteners, on opposite sides of the retainer. A pressure clamp with threaded fasteners is generally less preferred than a spring clip-type pressure clamp, but may be more suitable for certain applications.
Particularly for use with high-voltage semiconductor devices, the mounting system preferably includes a spacer of electrically insulating thermally conductive material for interposing between the heat spreader and the heat sink. The spacer is configured to facilitate thermal conduction from the semiconductor package to the heat sink, while isolating the semiconductor package and heat sink electrically. The electrically insulating thermally conductive material may be a ceramic material, such as alumina and beryllia, or less preferably, a plastic material. Particularly when a ceramic material is used, the spacer may be a regular polygonal slab of uniform thickness, such as a rectangular slab. The length and width of the spacer are preferably relatively large compared to its thickness to facilitate thermal conduction through the spacer. For example, a spacer having dimensions of about 0.5 by 0.75 inches may be about 0.06 inches thick. The retainer is preferably shaped to hold the spacer in place between the heat spreader of the semiconductor package and the heat sink.
The retainer further may include a retention feature for retaining the semiconductor package in the recess of the retainer. In an embodiment of the invention, the semiconductor package has a protruding flange with a mounting hole, and the retention feature has a corresponding protrusion for inserting into the hole of the protruding flange, to retain the semiconductor package in the recess of the retainer. In the alternative, or in addition, the retainer may include alignment features for aligning the retainer with respect to the heat sink. In an embodiment of the invention, the heat sink has at least two alignment holes in the sheet of thermally conductive material, and the alignment features are at least two protruding features disposed on a face of the retainer. Each of the at least two protruding features is configured for inserting into a corresponding one of the at least two alignment holes. Using the foregoing retention and alignment features, positive and secure alignment may be achieved between a retainer, semiconductor package, spacer, and heat sink in conjunction with a spring clip-type pressure clamp.
The block of non-conductive material for the rigid retainer is preferably a molded plastic material, such as an injection molded plastic. The plastic material may be any suitable polymer material that is non-conductive and hard enough to maintain its shape under the pressure of the pressure clamp at the anticipated operating temperatures. Using an inexpensive plastic material, nearly any desired retainer shape may be fabricated at low unit cost.
The retainer shape may be standardized to fit particular types of semiconductor packages. For a given semiconductor package and thickness of heat sink, the spring clip may also be standardized. Using standardized components of the mounting system, it should be possible to mount semiconductor packages to a variety of different heat sinks for a variety of different applications. This versatility eliminates the need for custom components except for the heat sink itself, enabling quicker and less expensive prototyping and manufacture of assembled device/heat sink assemblies. The heat sink may be stamped from flat stock for further cost reduction and ease of prototyping. The mounting system is more compact than prior-art mounting systems, and more easily assembled and disassembled. When used with a suitable insulating spacer, the mounting system also provides for safer mounting of high-voltage semiconductor devices while providing improved thermal performance.
A more complete understanding of the mounting system for a semiconductor device will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly.